1. Field of the Invention
This invention relates to charged capacitive transducers, and in particular to diaphragm configurations in electroacoustic speakers, where a diaphragm is directionally stiffened and spaced from stator elements by a securing structure.
2. Prior Art
Electrostatic loudspeakers are relatively simple in theory and structure. Basically, the components consist of (i) one or two rigid stators to which an audio voltage is applied and (ii) a flexible emitter diaphragm between or adjacent to the stators, which is usually biased with a high voltage for optimal performance. Typically, a planar diaphragm is stretched between the opposing stators and slightly spaced therefrom to provide a small air gap in which the diaphragm oscillates. This structure is sometimes called a push-pull transducer, because one stator is pushing while the other is pulling or releasing the diaphragm.
One of the advantages of the electrostatic loudspeaker is it has a diaphragm which is driven equally at all points of its surface, thereby providing a linear operation and minimizing breakup, harmonic distortion and phase differences. Because the diaphragm and stators can be very light and there is normally no magnet, as in electrodynamic speakers, electrostatic loudspeakers are typically very light for their size.
Electrostatic loudspeakers have been on the market since the late 1940s, but have only had limited use and availability because of technical problems. Some of the difficulties include the competing requirements for diaphragm tension, resonant frequency, bias voltage and diaphragm stability. Prior art electrostatic speakers also require a large surface area to produce low frequencies, and tend to develop undesirable levels of directivity and capacitive impedance at higher frequencies.
Tensioning of the diaphragm is a particularly challenging problem. Difficulties are encountered in applying and maintaining precise tension on the diaphragm to avoid distortion while obtaining an optimal range of frequency response. If the diaphragm is slightly too loose, distortion becomes apparent. If tension on the diaphragm is too tight, the low frequencies may be muted or lost. Thus, frequency response over a wide spectrum can be difficult.
Another key problem is that a speaker typically starts to fall off in amplitude at six decibels per octave with decreasing audio frequencies. The resonance frequency is usually exhibited after a substantial part of where the decibel drop-off occurs. At the resonant frequency of the transducer, a substantial amplitude peak is encountered followed by an even more severe amplitude drop-off of twelve additional decibels per octave. These amplitude drops make it difficult to maintain a consistent volume at lower frequencies. Some designers of electrostatic speakers have addressed the frequency range problem by employing different sized drivers, which adds to the cost, size and complexity of the speakers. Others have divided the diaphragm area into more easily handled sub-panels, which each have their own frequency response characteristics. U.S. Pat. No. 5,054,081 to West teaches an electrostatic transducer in which a number of stretched diaphragm sections are constructed and arranged so that each section has a resonant frequency that differs from that of the other diaphragm sections. However, the sensitivity problems associated with requiring precise tension on the diaphragm are still present.
What is needed is an electrostatic transducer that does not demand precise tensioning in order to obtain a wide frequency response. Moreover, an electrostatic transducer is needed that does not encounter significant variations in amplitude because of decibel drop-off and resonant frequency amplitude spikes. Further, An electrostatic transducer is needed that is lightweight, inexpensive and simple in construction. In addition, an electrostatic speaker that can be curved to provide desired directional characteristics would be advantageous.
It is an object of the present invention to provide an electroacoustic speaker with a broad band, high quality audio output.
It is a further object of this invention to provide an electrostatic speaker that is mechanically superior to prior art electrostatic speaker transducers.
It is also an object of the present invention to provide sufficient stiffness to the diaphragm to enable operable oscillations of the diaphragm, without requiring the diaphragm to be under tension.
It is another object of the present invention to provide a way to compensate for the amplitude drop-off, the resonant frequency spike of the electrostatic transducer, and to enhance the frequency response of the speaker.
It is yet another object to provide an electrostatic speaker which is light-weight, inexpensive and simple to construct.
In one preferred embodiment, an electroacoustic transducer includes at least one stator member having an operating surface positioned adjacent to an emitter diaphragm. The diaphragm is suspended and spaced a sufficient distance from the operating surface of the stator member to enable diaphragm oscillation in response to an applied signal voltage without incurring contact from the operating surface of the stator member. The diaphragm has at least one increased stiffness orientation which provides a directional stiffness along the diaphragm and within the emitter section to enable the emitter diaphragm to oscillate without applying tension in the direction of stiffness. A securing structure or clamp is applied to the diaphragm and the operating surface of the stator member to maintain the diaphragm in a fixed position relative to the stator.
In another preferred embodiment, a method is provided for generating audio output from an electroacoustic transducer. The method includes the step of positioning at least one stator member having an operating surface adjacent an emitter diaphragm. The second step is suspending an emitter diaphragm adjacent to and spaced a sufficient distance from the operating surface of the stator member. This enables diaphragm oscillation in an emitter section of the diaphragm in response to an applied signal voltage without incurring interfering contact with the operating surface of the stator member. The next step is configuring the diaphragm with at least one increased stiffness orientation to provide a primary directional stiffness along the diaphragm and within the emitter section to enable the emitter diaphragm to operably oscillate in the absence of tension applied along the stiffness orientation. The final step is positioning a securing structure between the diaphragm and the operating surface of the stator member to secure the diaphragm in a fixed position relative to the stator.
Other objects and features of the present invention will be apparent from the following detailed description, taken in combination with the following drawings.